The present invention relates to a high-speed centrifugal extractor capable of rapid liquid-liquid extraction by generating centrifugal force.
The high-speed centrifugal extractor can be used particularly for separating uranium and plutonium contained in spent nuclear fuel of nuclear fission products when reprocessing the spent nuclear fuel by a solvent extraction method (such as a Purex process), but is not limited to this application and can be applied widely to liquid-liquid extraction of any mixture comprising a heavy liquid and a light liquid.
A reprocessing method for spent nuclear fuel by employing the Purex process involves the steps of bringing a nitric acid solution (heavy liquid) containing uranium, plutonium and nuclear fission products into counter current flow contact with a tributyl phosphate (hereinafter called "TBP") diluted with hydrocarbon solution (light liquid) as an extraction solvent of uranium and plutonium in order to extract uranium and plutonium into the TBP from the nitric acid solution, then bringing once again this TBP into counter current flow contact with a new nitric acid solution in order to remove and wash the nuclear fission products that have been extracted in slight amounts into the TBP, and further bringing this washed TBP into counter current flow contact with a dilute nitric acid solution to accomplish a reverse extraction of uranium and plutonium in the TBP into the dilute nitric acid solution.
Extractors such as a mixer settler, a pulse column, and the like, have been generally used for facilitating the extraction, washing and reverse extraction processes described above. In the mixer settler, however, a sufficient residence time must be secured because natural gravitational force is utilized to separate both liquids inside the extractor due to the difference of specific gravity of the liquids, and for this reason TBP as the extraction agent is likely to be damaged by radioactive rays. In the pulse column, on the other hand, it is known in the art that the dispersion state deteriorates due to wettability of the perforated plate fitted inside the column. These are the technical problems to be solved in order to obtain a high decontamination factor and a stable operating condition. In order to increase the processing capacity, a floor area must be increased in the mixer settler while the diameter and height of the column must be increased in the pulse column. This means that the overall size of the extractors must be increased.
A high-speed centrifugal extractor has recently been developed as an extractor that solves the problems of the conventional extractors described above. The high-speed centrifugal extractor forcibly separates a mixture of the heavy liquid and light liquid into the heavy liquid and the light liquid by centrifugal force, and its typical construction is shown in FIG. 4 of the accompanying drawings. The high-speed centrifugal extractor fundamentally comprises a casing 1 and a cylindrical rotor 2 that is rotated at a high speed by a rotary shaft 3 inside the casing. The heavy liquid (such as a nitric acid solution) and a light liquid (such as TBP as an extraction agent) are supplied into a mixing chamber 6 at a lower part of the casing 1 from respective supply pipes 4 and 5. After the liquids are sufficiently mixed inside the mixing chamber 6 by an impeller 7 disposed at the lower end of the rotary shaft and rotating with it, the mixture is introduced into the rotor 2 through a center opening 9 of a rotor lower end plate 8. After the mixture is further stirred between the rotor end plate 8 and a baffle plate 10, the heavy liquid having a greater specific gravity is forced outwards while the light liquid having a smaller specific gravity remains inward of the heavy liquid due to the centrifugal force generated at the inner peripheral surface 2a of the rotor, and the liquids rise upwards on the rotor inner peripheral surface. Weirs 11 and 12 for facilitating selective extraction are disposed at the upper part inside the rotor 2 in order to separate the heavy liquid and the light liquid and to introduce them to respective outlets 13 and 14. The weir 11 for facilitating selective extraction of the heavy liquid has a heavy liquid draw port 11a that is open outwardly of the interface K between the outer heavy liquid phase and the inner light liquid phase, that is, on the side of the heavy liquid phase, and the heavy liquid passing through this draw port 11a flows over a plurality of weir plates 11b, 11c, 11d and is introduced into the heavy liquid outlet 13, and is discharged therefrom to a heavy liquid discharge port 16 through a heavy liquid collection chamber 15 (as represented by the solid line arrow in the drawing). On the other hand, the light liquid selection weir 12 has a light liquid draw port 12a that is open inwardly of the interface K between the heavy liquid phase and the light liquid phase, that is, on the side of the light liquid phase, and the light liquid flowing through this draw port 12a is introduced into a light liquid outlet 14, and is discharged therefrom to a light liquid discharge port 18 through a light liquid collection chamber 17 (as represented by dotted line arrow in the drawing).
Since the high-speed centrifugal extractor forcibly separates the heavy liquid and the light liquid by centrifugal force as described above, it provides the following advantages.
(1) Since mixing is performed at a high-speed, the extraction efficiency is high. PA1 (2) Since the contact time is extremely short, damage of the extraction agent by radioactive rays is minimal. PA1 (3) Since the quantity of the liquid residing inside the extractor is small, the hold up quantity of nuclear substances and radioactive substances is small. PA1 (4) The size of the extractor required in order to secure the same processing capacity as that of the conventional pulse column or mixer settler is extremely small. PA1 (5) The time required before the equilibrium state is reached is short so that the time required from the start to the finish of processing is extremely short and also the quantity of resulting waste liquor is extremely small.
In the centrifugal extractor of the type described above, sufficient centrifugal force will not act upon the mixed liquid introduced into the cylinder of the rotor 2 unless the mixed liquid rotates at a high speed with the revolution of the rotor. Therefore, in the conventional centrifugal extractor, a plurality of partitions 19 are disposed inside and in the longitudinal direction of the rotor and extend radially from the rotary shaft 3 to the rotor inner peripheral surface 2a, to divide the rotor cylinder into a plurality of chambers as shown in FIG. 5 and thus prevent the occurrence of slip between the liquid and the rotor inner peripheral surface 2a. (In FIG. 4, the partitions 19 are not shown.)
However, a plurality of partitions 19 must be firmly fixed between the rotary shaft 3 and the rotor inner peripheral surface 2a by welding or the like. Accordingly, disassembly and inspection inside the rotor cannot be made easily. Since the rotor 2 rotates at a high speed, the partitions 19 must be fitted in a well balanced condition and the production and assembly of the partitions 19 with an ordinary machining technique is rather difficult.
If the mixture of the heavy liquid and the light liquid introduced into the rotor 2 contains any solid particles having a greater specific gravity than that of the heavy liquid, these solid particles will be deposited on the inner peripheral surface 2a of the rotor due to the centrifugal force inside the rotor. The solid particles thus deposited cannot be removed easily and eventually, the entire rotor must be replaced.
The mixture of the heavy liquid and the light liquid formed inside the mixing chamber 6 at the lower part of the casing 1 is introduced into the rotor 2 by the pressure of the impeller 7 through the center opening 9 of the rotor lower end plate 8 and at this time, the mixed liquid cannot be directly fed into the upper part of the rotor by the action of the baffle plate 10. If the pressure generated by the impeller 7 is high, however, the mixed liquid is fed directly into the upper portion of the rotor without being separated, and the effective separation between the heavy liquid and the light liquid due to centrifugal force will not be made.